New Lipophilic 2-Amino-N,N′-dialkyl-4,5-dimethylimidazolium Cations: Synthesis, Structure, Properties, and Outstanding Thermal Stability in Alkaline Media

• Published in: Chemistry : a European journal 2009-09, Vol.15 (37), p.9477-9485
• Main Authors: Kunetskiy, Roman A., Císařová, Ivana, Šaman, David, Lyapkalo, Ilya M.
• Format: Article
• Language: English
• Subjects: aromatic stabilization; imidazolium cations; lipophilicity; phase-transfer catalysis; steric protection
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200901203

A series of new N,N′‐dialkyl‐4,5‐dimethylimidazolium cations possessing electron‐rich 2‐imidazolylidene‐ or phosphoranylidene‐amino substituents has been efficiently synthesized from common precursors, N,N′‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidenes. The new lipophilic salts obtained have been found to be highly stable towards strong alkali under both biphasic and homogeneous conditions. Their exceptional aqueous base resistance, which has hitherto only been seen with peralkylated polyaminophosphazenium cations, may be attributed to three factors: aromatic stabilization, efficient resonance charge delocalization, and steric protection of the exocyclic nitrogen linkage due to bulky lipophilic N‐alkyl substituents. Highly stable imidazolium cations: Efforts have been directed towards augmenting the stability and base resistance of substituted imidazolium cations, such as those depicted, through judicious combination of electronic and steric effects. By applying this design principle, novel imidazolium cations have been obtained that display exceptional robustness under strongly basic conditions at high temperatures.